Printed circuit board and method of reducing crosstalk in a printed circuit board

ABSTRACT

The embodiments of the present invention relate to an improved printed circuit board having additional rows of ground vias to reduce crosstalk in the board. A printed circuit board according to one embodiment of the present invention comprises a first row of vias and a second row of vias, each having a plurality of signal vias. The circuit board also comprises a plurality of rows of vias being coupled to a ground plane between the first row of signal vias and the second row of signal vias. According to one embodiment, the plurality of rows of vias being coupled to a ground plane comprise rows of vias having different sizes. Some of the vias are designed to receive a component, while others are generally smaller and designed to provide a return current path for the signal vias.

FIELD OF THE INVENTION

The present invention relates generally to printed circuit boards, andin particular, to a method of reducing crosstalk in a printed circuitboard.

BACKGROUND OF THE INVENTION

Current and future telecommunication switches make extensive use ofbackplane connectors. These connectors are used to join various systemcomponents of a communication switch together. Due to the strong desireon the part of the marketplace for high pin density, these connectorsare frequently constructed such that signal pairs are placed in closeproximity to one another. A problem that results from this arrangementis that the electric and magnetic fields of signals traversing throughthe connector interact with one another, causing an undesirablecrosstalk between the signals. As data rates continue to increase, theproblem of crosstalk will also increase.

High speed serial backplanes are a common media used in the creation ofnetwork switches. The backplane is a very thick printed wire board (PWB)or printed circuit board (PCB) with many internal layers whose surfaceis populated with connectors. The backplane is typically a passive boardthat serves to interconnect the various connectors on the surface of theboard. In general the arrangement of the pins is such that there arerows of pins in a ground-signal-signal-ground (GSSG) arrangement.Adjacent columns of these GSSG will interact both electrically andmagnetically. Line cards comprising additional printed circuit boardshaving dedicated integrated circuits mounted on them are coupled to thebackplane. The capabilities of the IC's coupled with the connectivity ofthe backplane form a complete switch that is capable routing digitaldata.

Crosstalk can occur in several ways. For high speed communication, twowires are used per data line. The wires themselves can support two formsof EM fields, an even mode and an odd mode. For high-speed digital datatransmission, differential signaling is used. Differential signalingcorresponds to the odd mode. The receiver in a high-speed differentialdata transmission system is designed to detect odd mode signals. Alongthe pair, changes in field configuration occur when there is a physicaltransition. This can occur for example when the striplines in thebackplane hit the backplane connector vias, or signals traveling in thevias hit the actual backplane connector. One impact of thesediscontinuities is mode conversion (i.e. when an even mode signal isconverted into an odd mode signal). A consequence of mode conversion isthat it is possible for even mode noise to be converted into an odd modesignal that will be detected at the receiver.

For a victim aggressor scenario where both the victim and aggressor aredifferential pairs, there are two primary ways for crosstalk to manifestitself on the receiver. Differential signals on the aggressor can affectthe odd mode of the victim. Also, even mode signals from the aggressorcouple to the victim and the victim pair converts the even mode signalinto an odd mode signal. Reducing the ability of modes to couple wouldsubstantially reduce the crosstalk.

Network switches enable the transfer of serial digital data within thissystem at data rates in excess of 1 Gbps (gigabit per second). Manypopular forms of encoding of the serial data result in signalfrequencies in excess of 1 GHz. At such high frequencies, crosstalk ofsignals becomes a barrier to effective error free communication. Asignificant source of crosstalk occurs within the vias of the backplaneand printed circuit boards.

Accordingly, there is a need for an improved printed circuit board andmethod of reducing crosstalk in a printed circuit board.

SUMMARY OF THE INVENTION

The embodiments of the present invention relate to an improved printedcircuit board having additional rows of ground vias to reduce crosstalkin the board. A printed circuit board according to one embodiment of thepresent invention comprises a first row of vias and a second row ofvias, each having a plurality of signal vias. The circuit board alsocomprises a plurality of rows of vias being coupled to a ground planebetween the first and second rows of signal vias. According to oneembodiment, the plurality of rows of vias being coupled to a groundplane comprise rows of vias having different sizes. Some of the vias aredesigned to receive a component, while others are generally smaller anddesigned to provide a return current path for the signal vias.

A method of reducing crosstalk in a printed circuit board is alsodescribed. The method generally comprises steps of providing a pluralityof rows of signal vias coupling predetermined leads of a plurality ofcomponents to signal vias; coupling ground leads of the plurality ofcomponents to a plurality of rows of ground vias; and providing a returnpath for signals on the signal vias by way of rows of ground viasadjacent to the rows of signal vias. A method for forming a printedcircuit board designed to reduce crosstalk is also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a printed circuit board assemblyaccording to an embodiment of the present invention;

FIG. 2 is a top plan view of the printed circuit board 102 of FIG. 1;

FIG. 3 is a cross sectional view of the printed circuit 102 taken atlines 3-3;

FIG. 4 is an expanded view of the via arrangement of section 202 of theprinted circuit board of FIG. 2;

FIG. 5 is a cross sectional view of the printed circuit 102 taken atlines 5-5;

FIG. 6 is an expanded view of the via arrangement of section 204 of theprinted circuit board of FIG. 2;

FIG. 7 is a cross sectional view of the printed circuit 102 taken atlines 7-7;

FIG. 8 is a top plan view of via arrangements of sections 202 and 204showing electrical fields according to the present invention;

FIG. 9 is a flow chart showing a method of reducing crosstalk in aprinted circuit board according to the present invention; and

FIG. 10 is a flow chart showing a method of forming a multi-layerprinted circuit board according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Turning now to FIG. 1, a perspective view of a printed circuit boardassembly according to an embodiment of the present invention is shown.In particular, a printed circuit board 102 comprising a backplane has aplurality of connectors 104. The plurality of connectors are adapted toreceive other printed circuit boards 106, such as by way ofcorresponding connectors 108 on the printed circuit boards 106. As willbe described more detail reference to later figures, the embodiments ofthe present invention enable reduced crosstalk in the printed circuitboard by providing additional ground vias which provide additionalreturn paths for signals. As shown in FIG. 2, the top plan view showsprinted circuit board 102 of FIG. 1 having a number of vias arranged onthe printed circuit board. The printed circuit board could be any typeof multi-layer printed circuit board having vias, and finds particularapplication in a backplane of a circuit board assembly for enabling thetransmission of high speed data signals. Cross sectional views of thevias taken at lines 3-3, lines 5-5 and lines 7-7 are shown in FIGS. 3, 5and 7, respectively. The arrangement of vias of sections 202 and 204 areshown in FIGS. 4 and 6, respectively.

Turning now to FIG. 3, a cross sectional view of the printed circuit 102is taken at lines 3-3. The ground vias are typically required by themanufacturer to provide mechanical support and allow a method for returncurrent to enter the connector. A plurality of layers 302, 304 and 306of the printed circuit board include a number of vias 308 and 310enabling the transmission of signals between the various layers. Ananti-pad 312 disconnects via 310 from layer 302. As can be seen, asingle trace 314 enables a signal to travel by way of the via 312. Areturn current travels by way of the via 308 to the layer 302.

Turning now to FIG. 4, an expanded view of the via arrangement ofsection 202 of the printed circuit board of FIG. 3 is shown. As can beseen, any two signal rows having signal vias 402 and 404, are positionbetween two ground rows having vias 406 and 408, for example. Such anarrangement provides a ground-signal-signal-ground (GSSG) arrangement.The cross sectional view of the printed circuit 102 taken at lines 5-5also shows the GSSG arrangement.

Turning now to FIG. 6, an expanded view of the via arrangement ofsection 204 of the printed circuit board of FIG. 3 is shown. As shown inFIG. 6, a modification to the board layout according to an embodiment ofthe present invention provides additional ground rows such that itreduces the ability of ground-signal-signal-ground (GSSG) columns tointeract with one another. The additional rows of ground vias (g) areplaced in the board to create a new return current path for signalstraversing the vias in the board. For example, signal vias 602 and 604,which are large enough to receive a lead of a component, such as aconnector, are on either end of a plurality of ground vias. Also, aconnector ground row (G) has a via 606 which is also large enough toreceive a lead of a component. Additional ground vias 608 and 610 arepositioned according to the present invention adjacent to the via rowshaving signal vias 602 and 604, respectively, and the connector groundrow (G) having via 606. As can be seen, three rows of ground vias arepositioned between two consecutive rows of signal vias. Although threerows of ground vias are shown, additional rows of ground vias could alsobe employed according to the present invention.

While providing increased immunity from crosstalk, the arrangement ofthe embodiment of FIG. 7 does not affect the routing density of theconnector. That is, the additional ground vias are preferably of asmaller diameter than the ground vias for receiving components on theboard. The additional ground vias provide the electrical function ofproviding a return path without requiring the space of the ground viarows (G) which are large enough to receive components, such asconnectors for receiving a mating connector of a separate printedcircuit board, or individual components. The cross sectional view of theprinted circuit 102 taken at lines 5-5 of 7 shows how the additionalground vias provide a closer return path for the signal vias. Althoughthe additional ground via rows (g) have vias which have a smallerdiameter than the signal vias or the vias of ground via rows (G), thevias of the additional ground via rows (g) could have any diameter,including a diameter of the signal vias or the vias of ground row vias(G). Smaller vias are preferably used to minimize the size requirementof the board. As will be described in greater detail in reference toFIG. 8, the benefit of the addition rows of ground vias is a reductionin the available loop area whereby the electric and magnetic fields ofan “aggressive via” can couple into a “victim via.”

Turning now to FIG. 8, a top plan view of via arrangements of section202 (which comprises a conventional arrangement of vias) and section 204(which comprises vias arranged according to FIG. 6) shows electricalfields around two adjacent signal vias, and how the embodiment of FIG. 6reduces the crosstalk in the printed circuit board. By altering theground placement, return current from the signal vias is forced to runcloser to the signal pairs, thereby reducing the available loop areawhereby the electric and magnetic fields of an “aggressor via” cancouple into a “victim via.” As the return current vias are now closer tosignal lines, a new differential pair is formed that is less susceptibleto crosstalk interference.

Although a via will support modes other than TEM, the following is afirst order explanation that assumes TEM fields within the via. The leftsides 802 and 804 of each pair are victims and the right sides 806 and808 of each pair are aggressors. The magnetic (H) field that would beassociated with an aggressor has a common mode excitation as shown. Allelectrical high speed signals are composed of a signal path and a returnpath. If a line of magnetic flux encircles a signal path and does notencircle a return path, a current is created in the signal path only.This imbalance between signal and return manifests itself as a voltage(i.e. the current induced times the impedance of the line). If a line ofmagnetic flux encircles both signal and return paths, this impresses acurrent equally on both signal and return. This is a common mode shiftwhich does not result in any sort of imbalance. For the via arrangementof section 202 on the left, lines 4 and 5 of magnetic flux fullyencircle the victim signal pair without also encircling a return path(i.e. a via of a ground row). This introduces a common mode current inthe victim pair. In the modified via arrangement of section 204 on theright, only line 4 encircles the victim signal pair without alsoencircling a return path. That is, line 5 encircles both signal andreturn paths for each of the aggressor and victim pairs because ofadditional rows of ground vias, and therefore does not introduce anycommon mode current in the victim pair. By having the return in closerto the signal lines, the potential for common mode crosstalk to occur isreduced. Because the via is a converter between even and odd modes, anyreduction in the amount of even mode noise that is coupled onto a pairwould lead to less noise available that can be converted intodifferential noise.

Turning now to FIG. 9 is a flow chart showing a method of reducingcrosstalk in a printed circuit board according to the present invention.A plurality of rows of signal vias are provided at a step 902.Predetermined leads of a plurality of components are coupled to signalvias of the plurality of rows of the signal vias at a step 904. Theplurality of rows of ground vias having vias of a diameter to receiveleads of the plurality of components are provided at a step 906. Groundleads of the plurality of components are coupled to a plurality of rowsof ground vias at a step 908. Rows of ground vias which have a diameterless than the diameter of the vias of the plurality of rows of groundvias are provided adjacent the rows of signal vias at a step 910. Areturn path for signals on the signal vias are provided by way of rowsof ground vias adjacent to the rows of signal vias at a step 912. Asecond printed circuit board is coupled to a component of the pluralityof components at a step 914. Finally, high speed communication signalsare provided by way of the printed circuit board at a step 916.

Turning now to FIG. 10, a flow chart shows a method of forming amulti-layer printed circuit board according to the present invention. Inparticular, a plurality of layers, each layer having traces for routingelectrical signals, are formed in a printed circuit board at a step1002. A first plurality of rows of vias are formed through the pluralityof layers at a step 1004. The vias of the first plurality rows have afirst diameter and transmit electrical signals by way of a plurality ofconnectors. A second plurality of rows of vias are formed through theplurality of layers at a step 1006. The vias of the second pluralityrows also have a first diameter and are coupled to the plurality ofconnectors to provide a ground connection. A third plurality of rows ofvias are formed through the plurality of layers at a step 1008. The viasof the third plurality of rows of vias have a second diameter smallerthan the first diameter. Finally, a return current path for current inthe first plurality of rows of vias is provided by way the thirdplurality of rows of vias at a step 1010.

It can therefore be appreciated that the new and novel printed circuitboard and method of reducing crosstalk in a printed circuit board hasbeen described. Although they can be used with any type of printedcircuit board, the embodiments of the present invention can be used forany type of backplane connector. As the drilling aspect ratio of PWBvendors improve this approach can find more and more application. Itwill be appreciated by those skilled in the art that, particular theteaching herein, numerous alternatives and equivalents will be seen toexist which incorporate the disclosed invention. As a result, theinvention is not to be limited by the foregoing embodiments, but only bythe following claims.

1. A printed circuit board assembly having signal vias and ground vias,said printed circuit board assembly comprising: a back plane printedcircuit board, said back plane printed circuit board comprising: aplurality of connectors; a pair of rows of vias having signal vias, saidsignal vias receiving leads of said plurality of connectors; a pair ofrows of vias having ground vias between said pair of rows having signalvias, said pair of rows of vias having ground vias located adjacent tosaid pair of rows of vias having signal vias; and a row of vias havingground vias between said pair of rows having ground vias and receivingother leads of said plurality of connectors; and a plurality of printedcircuit boards having corresponding connectors, said plurality ofprinted circuit boards being coupled to said plurality of connectors ofsaid back plane printed circuit board by way of said plurality ofcorresponding connectors.
 2. The printed circuit board assembly of claim1 wherein said plurality of printed circuit boards comprises a pluralityof integrated circuit packages.
 3. The printed circuit board assembly ofclaim 1 wherein said backplane printed circuit board and said pluralityof printed circuit boards form a switch for routing digital data.
 4. Theprinted circuit board assembly of claim 1 wherein said pair of rows ofvias having ground vias comprises vias having a smaller diameter thansaid signal vias.
 5. The printed circuit board assembly of claim 1wherein said pair of rows of vias having ground vias provides returncurrent paths for signals in said signal vias.